These compounds are used as synthesis intermediates for numerous applications relating, in general, to the field of fluorinated surfactant substances and, more particularly, bases for fire-extinguishing formulations, hydrophobic and oleophobic finishing agents for the treatment of textiles or paper, and more recently applications of a medical nature (contrast agents or oxygen transporters).
The linear perfluoroalkyl iodides are customarily obtained by telomerization of tetrafluoroethylene with pentafluoroethyl iodide C.sub.2 F.sub.5 I, which is in turn prepared by the action of iodine and iodine pentafluoride on tetrafluoroethylene in the presence of a catalyst. These two reactions may be coupled, as described in patent FR 1 385 682, but in the majority of cases C.sub.2 F.sub.5 I is prepared first and is then used in the telomerization. Branched perfluoroalkyl iodides can be obtained from a secondary perfluoroalkyl iodide such as heptafluoroisopropyl iodide CF.sub.3 CFICF.sub.3.
The telomerization reaction can be carried out in accordance with at least three methods, which differ essentially in the means of activation, which may be:
either free-radical activation using various peroxide initiators, as in the processes which are the subject of patents FR 2 035 913, FR 2 325 665 and U.S. Pat. No. 3,226,449, PA1 or catalytic activation involving the use of a redox system, as in the processes according to patents FR 2 028 781 and FR 2 098 335, PA1 or, finally, thermal activation, as in the processes which are the subject of patents FR 1 415 198 and U.S. Pat. No. 3,404,189.
In all of these processes, a more or less wide distribution of the various chain lengths is obtained and, even in the catalytically initiated processes which are reputed to be more selective, it is difficult to achieve a relatively narrow distribution for a telomer of extent j, j ranging from 2 to 5 and denoting the number of molecules or groups of tetrafluoroethylene which are telomerized with pentafluorethyl iodide or heptafluoroisopropyl iodide.
It is well known that this telomerization has the particular characteristic of giving products which may in turn function as telogen and may thus contribute to chain lengthening, a function which is almost exclusively performed by propagation reactions in the majority of telomerizations. In all of the text which follows, any perfluoroalkyl iodide may be considered as either a telomer or a telogen of extent i, C.sub.2 F.sub.5 I or CF.sub.3 CFICF.sub.3 being by definition only the telogen of extent 0.
In a large number of applications it is possible to use all of the telomers, or all at least of the fractions of extent i to j, j being an integer which is greater than or equal to i+1. On the other hand, certain applications necessitate the use of products which have a well-defined perfluorinated chain length.
Starting from a telogen of extent i, it is relatively easy to obtain the telomer of extent i+1 with a good selectivity, by reducing the degree of conversion by any suitable means (temperature, contact time, molar ratio telogen: C.sub.2 F.sub.4). For a telomer of extent j where j&gt;i+1, the problem is more difficult, and optimization is almost impossible without recycling some or all of the telomers of extent i+1 to j-1, with the risk of increasing the proportion of telomers with an extent greater than j, i.e. the fraction of unwanted, heavy products.
Patent Application EP 0 433 988 describes a more elaborate means of improving the productivity and of orienting the reaction towards the formation of specific compounds which have, in particular, well-defined carbon chain lengths. To achieve this objective, a part of the liquid reaction mixture is withdrawn from the second half of a tubular reactor and is reinjected, via a second loop, into the first half of the tube, such that the reaction space between the two points thus defined represents from 20 to 90 % of the overall reaction volume. On leaving the reactor the reaction mixture is fractionated; the heavy products of value are withdrawn from the apparatus and the light products and unused reactants are returned to the head of the reactor, where they are brought together with the feed of fresh reactants. In fact, if the sum of the products recycled via the first and second loops is taken into account, such a process involves a very large volume of recyclate in relation to the productivity of the apparatus, the mass of recycled products being from 80 to 200 times greater than the effective production, according to the examples given. Moreover, this process does not reduce the production of heavy telomers, since the ratios C.sub.8 F.sub.18 I:C.sub.10 F.sub.21 I+telomers of greater extents are equal to 2.1 and 1.45 for overall recycling rates of 200 and 80 respectively. The introduction of a second recycling loop increases the productivity of the system by about 20 to 30% but generates too high a proportion of heavy products which are of little or no commercial value, and the gains in selectivity for C.sub.8 F.sub.17 I can only be obtained at the expense of making the recycling circuits a disproportionate size.